Regarding the current electric vehicles, the most concerns and critical problems are the battery cost and battery life issues. Hence, the main target of the research and development is to improve battery life and to reduce battery cost. In terms of cost-performance ratio, the extension of the battery life may be a possible solution. The battery cells of a battery pack of an electric vehicle, whose battery life are generally about 5 to 10 years, cannot be utilized in producing high power to the electric vehicle when the battery cells have been used over the battery life due to becoming weak in their functionality and reliability. Although the used battery is not useless, the cells of the battery pack suffer from the problems of discrepancy in capacity, internal resistance, self-discharge rate, and energy conversion efficiency. In a case of supplying large current and having large power load, the discrepancy of the battery cells causes the entire battery pack's failure to provide the power required by the load.
In addition, lithium-ion batteries, among other types of battery employed in electric vehicles in general, has the advantages of high energy and power density. However, a battery management system is required to balance the voltages among the battery cells since the lithium-ion battery cells in series suffer from the discrepancy among the battery cells. When the cell with the lowest voltage (or the weakest cell) reaches an over-discharge voltage, the battery management system is to cut off the load (e.g., the motor of the electric vehicle) for protection, and the cut-off protection will cause the reduction in the effective cruising performance of the electric vehicle. However, if the system does not protect the weakest cell immediately and make the weakest cell avoid the problem of over-discharge, the battery life of the entire battery pack will be reduced.
In addition, it should be noted that the battery pack with large capacity and a large number of cells in series and/or in parallel differs from the application of the battery in consuming portable products. However, there is no perfect reliable battery management system available in the market, particularly directed to balance strategy among modules, with respect to the application of the battery pack with high voltage and a large number of cells in series. The above phenomenon will not cause too much impact on low-voltage or consumer electronic products. However, the impact of this phenomenon and their derivable problems on the large-scale battery applications, such as energy storage systems, electric buses, and so on, will be serious and cannot be ignored.
Thus, it is desirable to improve the battery management technology for the electric vehicle, so as to utilize the battery of the electric vehicle effectively.